I think the companion that 13 had the best dynamic with was Ryan. He was the one they made the most effort to actually open up and get close to, and Jodie and Tosin were the best at playing off each other, but we only got it in a few short bursts, a tiny handful of moments spread across two seasons. At first it did seem like Ryan was being set up as the lynchpin of this particular group. He was the POV character, Graham's grandson and Yaz's school friend. Ryan was the one connecting them all, not the Doctor. The seed of his training to be a mechanic also made it seem like he'd be something of an apprentice to 13, but that was dropped almost immediately, and accepting Graham quickly became the main focus of his arc in season 11.